The invention relates to a color display tube whose neck accommodates an electron gun system for emitting three co-planar electron beams towards a display screen, i.e. a central beam coinciding at least substantially with the axis of the display tube and two side beams located on both sides thereof, and a deflection unit which is secured coaxially around the display tube, which deflection unit comprises a system of line deflection coils which, when energized, deflects the electron beams in a first direction, and a system of field deflection coils which, when energized, deflects the electron beams in a direction transverse to the first direction.
In color display tubes of the in-line type the electron gun system is adapted to generate three co-planar electron beams which converge on the display screen. The deflection unit arranged around the display tube for deflecting the electron beams is used to deflect the electron beams in the one or the other direction from their normal undeflected straight path, so that the beams impinge upon selected pixels of the display screen so as to provide visual indications on this screen. By varying the magnetic deflection fields in a suitable manner, the electron beams can be moved up or down and to the left or the right across the (vertically arranged) display screen. By simultaneously varying the intensity of the beams, a visual presentation of information or a picture can be formed on the display screen. The deflection unit secured around the cone portion of the display tube comprises two systems of deflection coils so as to be able to deflect the electron beams in two directions which are transverse to each other. Each system comprises two coils which are arranged at opposite sides of the neck of the tube, which systems are 90.degree. offset around the neck of the tube. Upon energization, the two systems of deflection coils produce orthogonal deflection fields.
The fields are essentially perpendicular to the path of the non-deflected electron beams. A cylindrical core of magnetizable material, which surrounds the system of line deflection coils if it is of the saddle type, is generally used to concentrate the deflection fields and to increase the flux density in the deflection region.
The deflection coils may be of the saddle type, or of the type which is toroidally wound on the annular core (particularly the field deflection coils).
After mounting a deflection unit comprising field deflection coils and line deflection coils on the display tube for which the unit is intended, twist errors sometimes appear to occur in operation. A twist error is understood to means the parabolic convergence error of the red and blue beams in the Y direction which is produced on rotation of the side beam (RBY error). This problem is not new, but it becomes urgent if very strict requirements are imposed on the convergence performance, as in CMT and HDTV applications.